Front electrode for use in photovoltaic device and method of making same

ABSTRACT

This invention relates to a front electrode/contact for use in an electronic device such as a photovoltaic device. In certain example embodiments, the front electrode of a photovoltaic device or the like includes a multilayer coating including at least one transparent conductive oxide (TCO) layer (e.g., of or including a material such as tin oxide, ITO, zinc oxide, or the like) and/or at least one conductive substantially metallic IR reflecting layer (e.g., based on silver, gold, or the like). In certain example instances, the multilayer front electrode coating may include one or more conductive metal(s) oxide layer(s) and one or more conductive substantially metallic IR reflecting layer(s) in order to provide for reduced visible light reflection, increased conductivity, cheaper manufacturability, and/or increased infrared (IR) reflection capability. In certain example embodiments, the front electrode acts as not only a transparent conductive front contact/electrode but also a short pass filter that allows an increased amount of photons having high energy (such as in visible and near infra-red regions of the spectrum) into the active region or absorber of the photovoltaic device.

This application is a continuation-in-part (CIP) of each of U.S. Ser.Nos. 12/068,117, filed Feb. 1, 2008, 11/984,092, filed Nov. 13, 2007,11/987,664, filed Dec. 3, 2007, 11/898,641, filed Sep. 13, 2007,11/591,668, filed Nov. 2, 2006, and 11/790,812, filed Apr. 27, 2007, theentire disclosures of which are all hereby incorporated herein byreference.

Certain embodiments of this invention relate to a photovoltaic deviceincluding an electrode such as a front electrode/contact. In certainexample embodiments of this invention, the front electrode is of orincludes a transparent conductive coating (TCC) having a plurality oflayers, and may be provided on a surface of a front glass substrateopposite to a patterned surface of the substrate. The TCC may act toenhance transmission in selected PV active regions of the visible andnear IR spectrum, while substantially rejecting and/or blockingundesired IR thermal energy from certain other areas of the spectrum.

In certain example embodiments, the front electrode of the photovoltaicdevice includes a multi-layer coating (or TCC) having at least oneinfrared (IR) reflecting and conductive substantially metallic layer ofor including silver, gold, or the like, and possibly at least onetransparent conductive oxide (TCO) layer (e.g., of or including amaterial such as tin oxide, zinc oxide, or the like). In certain exampleembodiments, the multilayer front electrode coating is designed torealize one or more of the following advantageous features: (a) reducedsheet resistance and thus increased conductivity and improved overallphotovoltaic module output power; (b) increased reflection of infrared(IR) radiation thereby reducing the operating temperature of thephotovoltaic module so as to increase module output power; (c) reducedreflection and/or increased transmission of light in the region of fromabout 400-700 nm, 450-700 nm, and/or 450-600 nm, which leads toincreased photovoltaic module output power; (d) reduced total thicknessof the front electrode coating which can reduce fabrication costs and/ortime; (e) improved or enlarged process window in forming the TCOlayer(s) because of the reduced impact of the TCO's conductivity on theoverall electric properties of the module given the presence of thehighly conductive substantially metallic IR reflecting layer(s); and/or(f) reduced risk of thermal stress caused module breakage by reflectingsolar thermal energy and reducing temperature difference across themodule.

BACKGROUND AND SUMMARY OF EXAMPLE EMBODIMENTS OF INVENTION

Photovoltaic devices are known in the art (e.g., see U.S. Pat. Nos.6,784,361, 6,288,325, 6,613,603, and 6,123,824, the disclosures of whichare hereby incorporated herein by reference). Amorphous siliconphotovoltaic devices, for example, include a front electrode or contact.Typically, the transparent front electrode is made of a pyrolytictransparent conductive oxide (TCO) such as zinc oxide or tin oxideformed on a substrate such as a glass substrate. In many instances, thetransparent front electrode is formed of a single layer using a methodof chemical pyrolysis where precursors are sprayed onto the glasssubstrate at approximately 400 to 600 degrees C. Typical pyroliticfluorine-doped tin oxide TCOs as front electrodes may be about 400 nmthick, which provides for a sheet resistance (R_(s)) of about 15ohms/square. To achieve high output power, a front electrode having alow sheet resistance and good ohm-contact to the cell top layer, andallowing maximum solar energy in certain desirable ranges into theabsorbing semiconductor film, are desired.

Unfortunately, photovoltaic devices (e.g., solar cells) with only suchconventional TCO front electrodes suffer from the following problems.

First, a pyrolitic fluorine-doped tin oxide TCO about 400 nm thick asthe entire front electrode has a sheet resistance (R_(s)) of about 15ohms/square which is rather high for the entire front electrode. A lowersheet resistance (and thus better conductivity) would be desired for thefront electrode of a photovoltaic device. A lower sheet resistance maybe achieved by increasing the thickness of such a TCO, but this willcause transmission of light through the TCO to drop thereby reducingoutput power of the photovoltaic device.

Second, conventional TCO front electrodes such as pyrolytic tin oxideallow a significant amount of infrared (IR) radiation to passtherethrough thereby allowing it to reach the semiconductor or absorbinglayer(s) of the photovoltaic device. This IR radiation causes heat whichincreases the operating temperature of the photovoltaic device therebydecreasing the output power thereof.

Third, conventional TCO front electrodes such as pyrolytic tin oxidetend to reflect a significant amount of light in the region of fromabout 400-700 nm, or 450-700 nm, so that less than about 80% of usefulsolar energy reaches the semiconductor absorbing layer; this significantreflection of visible light is a waste of energy and leads to reducedphotovoltaic module output power. Due to the TCO absorption andreflections of light which occur between the TCO (n about 1.8 to 2.0 at550 nm) and the thin film semiconductor (n about 3.0 to 4.5), andbetween the TCO and the glass substrate (n about 1.5), the TCO coatedglass at the front of the photovoltaic device typically allows less than80% of the useful solar energy impinging upon the device to reach thesemiconductor film which converts the light into electric energy.

Fourth, the rather high total thickness (e.g., 400 nm) of the frontelectrode in the case of a 400 nm thick tin oxide TCO, leads to highfabrication costs.

Fifth, the process window for forming a zinc oxide or tin oxide TCO fora front electrode is both small and important. In this respect, evensmall changes in the process window can adversely affect conductivity ofthe TCO. When the TCO is the sole conductive layer of the frontelectrode, such adverse affects can be highly detrimental.

Thus, it will be appreciated that there exists a need in the art for animproved front electrode for a photovoltaic device that can solve oraddress one or more of the aforesaid five problems.

In certain example embodiments of this invention, there is provided afront electrode structure for a photovoltaic device, the front electrodestructure comprising: a front substantially transparent glass substrate;a first layer comprising one or more of silicon nitride, silicon oxide,silicon oxynitride, and/or tin oxide; a second layer comprising one ormore of titanium oxide and/or niobium oxide, wherein at least the firstlayer is located between the front substrate and the second layer; athird layer comprising zinc oxide and/or zinc aluminum oxide; aconductive layer comprising silver, wherein at least the third layer isprovided between the conductive layer comprising silver and the secondlayer; a layer comprising an oxide of Ni and/or Cr; a transparentconductive oxide (TCO) layer comprising indium tin oxide providedbetween the layer comprising the oxide of Ni and/or Cr and a transparentconductive oxide (TCO) layer comprising tin oxide; and wherein a layerstack comprising said first layer, said second layer, said third layer,said conductive layer comprising silver, said layer comprising the oxideof Ni and/or Cr, said TCO layer comprising indium tin oxide, and saidTCO comprising tin oxide, is provided on an interior surface of thefront glass substrate facing the semiconductor film of the photovoltaicdevice.

In certain example embodiments of this invention, the front electrode ofa photovoltaic device includes a transparent conductive coating (TCC)having a plurality of layers, and is provided on a surface of a frontglass substrate opposite to a patterned surface of the substrate. Incertain example embodiments, the patterned (e.g., etched) surface of thefront transparent glass substrate faces incoming light, whereas the TCCis provided on the opposite surface of the substrate facing thesemiconductor film of the photovoltaic (PV) device. The patterned firstor front surface of the glass substrate reduces reflection loss ofincident solar flux and increases the absorption of photon(s) in thesemiconductor film through scattering, refraction and diffusion.

In certain example embodiments, the TCC of the front electrode may becomprise a multilayer coating including at least one conductivesubstantially metallic IR reflecting layer (e.g., based on silver, gold,or the like), and optionally at least one transparent conductive oxide(TCO) layer (e.g., of or including a material such as tin oxide, zincoxide, or the like). In certain example instances, the multilayer frontelectrode coating may include a plurality of TCO layers and/or aplurality of conductive substantially metallic IR reflecting layersarranged in an alternating manner in order to provide for reducedvisible light reflections, increased conductivity, increased IRreflection capability, and so forth.

In certain example embodiments of this invention, a multilayer frontelectrode coating may be designed to realize one or more of thefollowing advantageous features: (a) reduced sheet resistance (R_(s))and thus increased conductivity and improved overall photovoltaic moduleoutput power; (b) increased reflection of infrared (IR) radiationthereby reducing the operating temperature of the photovoltaic module soas to increase module output power; (c) reduced reflection and increasedtransmission of light in the region(s) of from about 400-700 nm, 450-700nm, or 450-600 nm which leads to increased photovoltaic module outputpower; (d) reduced total thickness of the front electrode coating whichcan reduce fabrication costs and/or time; (e) an improved or enlargedprocess window in forming the TCO layer(s) because of the reduced impactof the TCO's conductivity on the overall electric properties of themodule given the presence of the highly conductive substantiallymetallic layer(s); and/or (f) reduced risk of thermal stress causedmodule breakage by reflecting solar thermal energy and reducingtemperature difference across the module.

In certain example embodiments of this invention, there is provided aphotovoltaic device comprising: a front glass substrate; an activesemiconductor film; an electrically conductive and substantiallytransparent front electrode located between at least the front glasssubstrate and the semiconductor film; wherein the substantiallytransparent front electrode comprises, moving away from the front glasssubstrate toward the semiconductor film, at least a first substantiallytransparent conductive substantially metallic infrared (IR) reflectinglayer comprising silver and/or gold, and a first transparent conductiveoxide (TCO) film located between at least the IR reflecting layer andthe semiconductor film; and wherein the front electrode is provided onan interior surface of the front glass substrate facing thesemiconductor film, and an exterior surface of the front glass substratefacing incident light is textured so as to reduce reflection loss ofincident solar flux and increase absorption of photons in thesemiconductor film, especially when the sunlight coming at a tintedangle.

In certain example embodiments of this invention, there is provided aphotovoltaic device comprising: a front glass substrate; a semiconductorfilm; a substantially transparent front electrode located between atleast the front glass substrate and the semiconductor film; wherein thesubstantially transparent front electrode comprises, moving away fromthe front glass substrate toward the semiconductor film, at least afirst substantially transparent layer that may or may not be conductive,a substantially metallic infrared (IR) reflecting layer comprisingsilver and/or gold, and a first transparent conductive oxide (TCO) filmlocated between at least the IR reflecting layer and the semiconductorfilm.

BRIEF DESCRIPTION OF. THE DRAWINGS

FIG. 1 is a cross sectional view of an example photovoltaic deviceaccording to an example embodiment of this invention.

FIG. 2 is a refractive index (n) versus wavelength (nm) graphillustrating refractive indices (n) of glass, a TCO film, silver thinfilm, and hydrogenated silicon (in amorphous, micro- or poly-crystallinephase).

FIG. 3 is a percent transmission (T %) versus wavelength (nm) graphillustrating transmission spectra into a hydrogenated Si thin film of aphotovoltaic device comparing examples of this invention versus acomparative example (TCO reference); this shows that the examples ofthis invention (Examples 1, 2 and 3) have increased transmission in theapproximately 450-700 nm wavelength range and thus increasedphotovoltaic module output power, compared to the comparative example(TCO reference).

FIG. 4 is a percent reflection (R %) versus wavelength (nm) graphillustrating reflection spectra from a hydrogenated Si thin film of aphotovoltaic device comparing the examples of this invention (Examples1, 2 and 3 referred to in FIG. 3) versus a comparative example (TCOreference referred to in FIG. 3); this shows that the example embodimentof this invention have increased reflection in the IR range, therebyreducing the operating temperature of the photovoltaic module so as toincrease module output power, compared to the comparative example.Because the same Examples 1-3 and comparative example (TCO reference)are being referred to in FIGS. 3 and 4, the same curve identifiers usedin FIG. 3 are also used in FIG. 4.

FIG. 5 is a cross sectional view of the photovoltaic device according toExample 1 of this invention.

FIG. 6 is a cross sectional view of the photovoltaic device according toExample 2 of this invention.

FIG. 7 is a cross sectional view of the photovoltaic device according toExample 3 of this invention.

FIG. 8 is a cross sectional view of the photovoltaic device according toanother example embodiment of this invention.

FIG. 9 is a cross sectional view of the photovoltaic device according toanother example embodiment of this invention.

FIG. 10 is a cross sectional view of the photovoltaic device accordingto another example embodiment of this invention.

FIG. 11 is a cross sectional view of the photovoltaic device accordingto another example embodiment of this invention.

FIG. 12 is a measured transmission (T) and reflection (R) (% from firstsurface 1 a) spectra, versus wavelength (nm), showing results from.Example 4 (having a 10 ohms/sq. Ag-based TCC coating a front glasssubstrate with a textured surface).

FIG. 13 is a transmission ratio versus wavelength (nm) graphillustrating results according to Example 4 (compared to a comparativeexample).

FIG. 14 is a percent transmission (T %) versus wavelength (nm) graphillustrating transmission spectra into an a-Si cell of a photovoltaicdevice, showing results of Example 5 of this invention having a texturedfront surface of the front glass substrate.

FIG. 15 is a percent transmission (T %) versus wavelength (nm) graphillustrating transmission spectra into a CdS/CdTe cell of a photovoltaicdevice comparing Example 4 of this invention (having a textured frontsurface of the front glass substrate) versus comparative examples; thisshows that the Example 4 of this invention realized increasedtransmission in the approximately 500-700 nm wavelength range and thusincreased photovoltaic module output power, compared to the comparativeexample without the etched front surface (x dotted line) and thecomparative example of the conventional TCO superstrate (o solid line).

FIG. 16 is a cross sectional view of the photovoltaic device accordingto another example embodiment of this invention.

FIG. 17 is a percent transmission (T %) versus wavelength (nm) graphillustrating transmission spectra into an a-Si cell of a photovoltaicdevice comparing the FIG. 16 embodiment of this invention versus acomparative example; this shows that the FIG. 16 embodiment of thisinvention (e.g., T-9 curve in FIG. 17) realized increased transmissionin the approximately 500-700 nm wavelength range and thus increasedphotovoltaic module output power, compared to the comparative example(X-marked curve in FIG. 17).

FIG. 18 is a percent transmission (T %) versus wavelength (nm) graphillustrating transmission spectra into a CdS/CdTe cell of a photovoltaicdevice comparing the FIG. 16 embodiment of this invention versus acomparative example; this shows that the FIG. 16 embodiment of thisinvention (e.g., T-Ag curve in FIG. 18) realized increased transmissionin the approximately 500-700 nm wavelength range and thus increasedphotovoltaic module output power, compared to the comparative example(X-marked curve in FIG. 18).

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

Referring now more particularly to the figures in which like referencenumerals refer to like parts/layers in the several views.

Photovoltaic devices such as solar cells convert solar radiation intousable electrical energy. The energy conversion occurs typically as theresult of the photovoltaic effect. Solar radiation (e.g., sunlight)impinging on a photovoltaic device and absorbed by an active region ofsemiconductor material (e.g., a semiconductor film including one or moresemiconductor layers such as a-Si layers, the semiconductor sometimesbeing called an absorbing layer or film) generates electron-hole pairsin the active region. The electrons and holes may be separated by anelectric field of a junction in the photovoltaic device. The separationof the electrons and holes by the junction results in the generation ofan electric current and voltage. In certain example embodiments, theelectrons flow toward the region of the semiconductor material havingn-type conductivity, and holes flow toward the region of thesemiconductor having p-type conductivity. Current can flow through anexternal circuit connecting the n-type region to the p-type region aslight continues to generate electron-hole pairs in the photovoltaicdevice.

In certain example embodiments, single junction amorphous silicon (a-Si)photovoltaic devices include three semiconductor layers. In particular,a p-layer, an n-layer and an i-layer which is intrinsic. The amorphoussilicon film (which may include one or more layers such as p, n and itype layers) may be of hydrogenated amorphous silicon in certaininstances, but may also be of or include hydrogenated amorphous siliconcarbon or hydrogenated amorphous silicon germanium, or the like, incertain example embodiments of this invention. For example and withoutlimitation, when a photon of light is absorbed in the i-layer it givesrise to a unit of electrical current (an electron-hole pair). The p andn-layers, which contain charged dopant ions, set up an electric fieldacross the i-layer which draws the electric charge out of the i-layerand sends it to an optional external circuit where it can provide powerfor electrical components. It is noted that while certain exampleembodiments of this invention are directed toward amorphous-siliconbased photovoltaic devices, this invention is not so limited and may beused in conjunction with other types of photovoltaic devices in certaininstances including but not limited to devices including other types ofsemiconductor material, single or tandem thin-film solar cells, CdSand/or CdTe (including CdS/CdTe) photovoltaic devices, polysiliconand/or microcrystalline Si photovoltaic devices, and the like.

In certain embodiments of this invention, the front electrode of the PVdevice is of or includes a transparent conductive coating (TCC) having aplurality of layers, and is provided on a surface of a front glasssubstrate opposite to a patterned surface of the substrate. In certainexample embodiments, the patterned (e.g., etched) surface of the fronttransparent glass substrate faces incoming light, whereas the TCC isprovided on the opposite surface of the substrate facing thesemiconductor film of the photovoltaic (PV) device. The patterned firstor front surface of the glass substrate reduces reflection loss ofincident solar flux and increases the absorption of photon(s) in thesemiconductor film through scattering, refraction and diffusion,especially when the sunlight coming at a tinted angle. The TCC may actto enhance transmission in selected PV active regions of the visible andnear IR spectrum, while substantially rejecting and/or blockingundesired IR thermal energy from certain other areas of the spectrum. Incertain example embodiments of this invention, the surface of the fronttransparent glass substrate on which the front electrode or TCC isprovided may be flat or substantially flat (not patterned), whereas inalternative example embodiments it may also be patterned.

FIG. 1 is a cross sectional view of a photovoltaic device according toan example embodiment of this invention. The photovoltaic deviceincludes transparent front glass substrate 1 (other suitable materialmay also be used for the substrate instead of glass in certaininstances), optional dielectric layer(s) 2, multilayer front electrode3, active semiconductor film 5 of or including one or more semiconductorlayers (such as pin, pn, pinpin tandem layer stacks, or the like), backelectrode/contact 7 which may be of a TCO or a metal, an optionalencapsulant 9 or adhesive of a material such as ethyl vinyl acetate(EVA) or the like, and an optional superstrate 11 of a material such asglass. Of course, other layer(s) which are not shown may also beprovided in the device. Front glass substrate 1 and/or rear superstrate(substrate) 11 may be made of soda-lime-silica based glass in certainexample embodiments of this invention; and it may have low iron contentand/or an antireflection coating thereon to optimize transmission incertain example instances. While substrates 1, 11 may be of glass incertain example embodiments of this invention, other materials such asquartz, plastics or the like may instead be used for substrate(s) 1and/or 11. Moreover, superstrate 11 is optional in certain instances.Glass 1 and/or 11 may or may not be thermally tempered and/or patternedin certain example embodiments of this invention. Additionally, it willbe appreciated that the word “on” as used herein covers both a layerbeing directly on and indirectly on something, with other layerspossibly being located therebetween.

Dielectric layer(s) 2 may be of any substantially transparent materialsuch as a metal oxide and/or nitride which has a refractive index offrom about 1.5 to 2.5, more preferably from about 1.6 to 2.5, morepreferably from about 1.6 to 2.2, more preferably from about 1.6 to 2.0,and most preferably from about 1.6 to 1.8. However, in certainsituations, the dielectric layer 2 may have a refractive index (n) offrom about 2.3 to 2.5. Example materials for dielectric layer 2 includesilicon oxide, silicon nitride, silicon oxynitride, zinc oxide, tinoxide, titanium oxide (e.g., TiO₂), aluminum oxynitride, aluminum oxide,or mixtures thereof. Dielectric layer(s) 2 functions as a barrier layerin certain example embodiments of this invention, to reduce materialssuch as sodium from migrating outwardly from the glass substrate 1 andreaching the IR reflecting layer(s) and/or semiconductor. Moreover,dielectric layer 2 is material having a refractive index (n) in therange discussed above, in order to reduce visible light reflection andthus increase transmission of visible light (e.g., light from about400-700 nm, 450-700 nm and/or 450-600 nm) through the coating and intothe semiconductor 5 which leads to increased photovoltaic module outputpower.

Still referring to FIG. 1, multilayer front electrode 3 in the exampleembodiment shown in FIG. 1, which is provided for purposes of exampleonly and is not intended to be limiting, includes from the glasssubstrate 1 outwardly first transparent conductive oxide (TCO) ordielectric layer 3 a, first conductive substantially metallic IRreflecting layer 3 b, second TCO 3 c, second conductive substantiallymetallic IR reflecting layer 3 d, third TCO 3 e, and optional bufferlayer 3 f. Optionally, layer 3 a may be a dielectric layer instead of aTCO in certain example instances and serve as a seed layer for the layer3 b. This multilayer film 3 makes up the front electrode in certainexample embodiments of this invention. Of course, it is possible forcertain layers of electrode 3 to be removed in certain alternativeembodiments of this invention (e.g., one or more of layers 3 a, 3 c, 3 dand/or 3 e may be removed), and it is also possible for additionallayers to be provided in the multilayer electrode 3. Front electrode 3may be continuous across all or a substantial portion of glass substrate1, or alternatively may be patterned into a desired design (e.g.,stripes), in different example embodiments of this invention. Each oflayers/films 1-3 is substantially transparent in certain exampleembodiments of this invention.

First and second conductive substantially metallic IR reflecting layers3 b and 3 d may be of or based on any suitable IR reflecting materialsuch as silver, gold, or the like. These materials reflect significantamounts of IR radiation, thereby reducing the amount of IR which reachesthe semiconductor film 5. Since IR increases the temperature of thedevice, the reduction of the amount of IR radiation reaching thesemiconductor film 5 is advantageous in that it reduces the operatingtemperature of the photovoltaic module so as to increase module outputpower. Moreover, the highly conductive nature of these substantiallymetallic layers 3 b and/or 3 d permits the conductivity of the overallelectrode 3 to be increased. In certain example embodiments of thisinvention, the multilayer electrode 3 has a sheet resistance of lessthan or equal to about 12 ohms/square, more preferably less than orequal to about 9 ohms/square, and even more preferably less than orequal to about 6 ohms/square. Again, the increased conductivity (same asreduced sheet resistance) increases the overall photovoltaic moduleoutput power, by reducing resistive losses in the lateral direction inwhich current flows to be collected at the edge of cell segments. It isnoted that first and second conductive substantially metallic IRreflecting layers 3 b and 3 d (as well as the other layers of theelectrode 3) are thin enough so as to be substantially transparent tovisible light. In certain example embodiments of this invention, firstand/or second conductive substantially metallic IR reflecting layers 3 band/or 3 d are each from about 3 to 12 nm thick, more preferably fromabout 5 to 10 nm thick, and most preferably from about 5 to 8 nm thick.In embodiments where one of the layers 3 b or 3 d is not used, then theremaining conductive substantially metallic IR reflecting layer may befrom about 3 to 18 nm thick, more preferably from about 5 to 12 nmthick, and most preferably from about 6 to 11 nm thick in certainexample embodiments of this invention. These thicknesses are desirablein that they permit the layers 3 b and/or 3 d to reflect significantamounts of IR radiation, while at the same time being substantiallytransparent to visible radiation which is permitted to reach thesemiconductor 5 to be transformed by the photovoltaic device intoelectrical energy. The highly conductive IR reflecting layers 3 b and 3d attribute to the overall conductivity of the electrode 3 much morethan the TCO layers; this allows for expansion of the process window(s)of the TCO layer(s) which has a limited window area to achieve both highconductivity and transparency.

First, second, and third TCO layers 3 a, 3 c and 3 e, respectively, maybe of any suitable TCO material including but not limited to conduciveforms of zinc oxide, zinc aluminum oxide, tin oxide, indium-tin-oxide,indium zinc oxide (which may or may not be doped with silver), or thelike. These layers are typically substoichiometric so as to render themconductive as is known in the art. For example, these layers are made ofmaterial(s) which gives them a resistance of no more than about 10ohm-cm (more preferably no more than about 1 ohm-cm, and most preferablyno more than about 20 mohm-cm). One or more of these layers may be dopedwith other materials such as fluorine, aluminum, antimony or the like incertain example instances, so long as they remain conductive andsubstantially transparent to visible light. In certain exampleembodiments of this invention, TCO layers 3 c and/or 3 e are thickerthan layer 3 a (e.g., at least about 5 nm, more preferably at leastabout 10, and most preferably at least about 20 or 30 nm thicker). Incertain example embodiments of this invention, TCO layer 3 a is fromabout 3 to 80 nm thick, more preferably from about 5-30 nm thick, withan example thickness being about 10 nm. Optional layer 3 a is providedmainly as a seeding layer for layer 3 b and/or for antireflectionpurposes, and its conductivity is not as important as that of layers 3b-3 e (thus, layer 3 a may be a dielectric instead of a TCO in certainexample embodiments). In certain example embodiments of this invention,TCO layer 3 c is from about 20 to 150 nm thick, more preferably fromabout 40 to 120 nm thick, with an example thickness being about 74-75nm. In certain example embodiments of this invention, TCO layer 3 e isfrom about 20 to 180 nm thick, more preferably from about 40 to 130 nmthick, with an example thickness being about 94 or 115 nm. In certainexample embodiments, part of layer 3 e, e.g., from about 1-25 nm or 5-25nm thick portion, at the interface between layers 3 e and 5 may bereplaced with a low conductivity high refractive index (n) film 3 f suchas titanium oxide to enhance transmission of light as well as to reduceback diffusion of generated electrical carriers; in this way performancemay be further improved.

In certain example embodiments, outer surface 1 a of the fronttransparent glass substrate 1 is textured (e.g., etched and/orpatterned). Herein, the user of the word “patterned” covers etchedsurfaces, and the use of the word “etched” covers patterned surfaces.The textured surface 1 a of the glass substrate 1 may have a prismaticsurface, a matte finish surface, or the like in different exampleembodiments of this invention. The textured surface 1 a of the glasssubstrate 1 may have peaks and valleys defined therein with inclinedportions interconnecting the peaks and valleys (e.g., see FIG. 1). Thefront major surface of the substrate 1 may be etched (e.g., via HFetching using HF etchant or the like) or patterned via roller(s) or thelike during glass manufacture in order to form a textured (and/orpatterned) surface 1 a. In certain example embodiments, the patterned(e.g., etched) surface 1 a of the front transparent glass substrate 1faces incoming light (see the sun in the figures), whereas the TCC 3 isprovided on the opposite surface 1 b of the substrate facing thesemiconductor film 5 of the photovoltaic (PV) device. The patternedfirst or front surface 1 a of the glass substrate 1 reduces reflectionloss of incident solar flux and increases the absorption of photon(s) inthe semiconductor film 5 through scattering, refraction and/ordiffusion. The transmission of solar flux into the photovoltaicsemiconductor 5 can be further improved by using the patterned/etchedsurface 1 a of the front glass substrate 1 in combination with theAg-based TCC 3 as shown in FIGS. 1 and 5-11. The patterned and/or etchedsurface 1 a results in an effective low index layer due to theintroduction of the void(s), and acts as an antireflection coating.Compared to a smooth front surface of the front substrate, the patternedand/or etched surface 1 a provides the following example advantages: (a)reduced reflection from the first surface 1 a, especially at obliqueincident angles, due to light trapping and hence increased solar fluxinto solar cells, and (b) increased optical path of light in thesemiconductor 5 thereby resulting in increased photovoltaic current.This may be applicable to embodiments of FIGS. 1 and 5-11 in certaininstances.

In certain example embodiments of this invention, average surfaceroughness on surface 1 a of the front glass substrate is from about 0.1μm to 1 mm, more preferably from about 0.5-20 μm, more preferably fromabout 1-10 μm, and most preferably from about 2-8 μm. Too large of asurface roughness value could lead to much dirt collection on the frontof the substrate 1, whereas too little of a roughness value on surface 1a could lead to not enough transmission increase. This surface roughnessat 1 a may be appliable to any embodiment discussed herein. Theprovision of such surface roughness on the surface 1 a of the substrateis also advantageous in that it can avoid the need for a separate ARcoating on the front glass substrate 1 in certain example embodiments ofthis invention.

In certain example embodiments, the interior or second surface 1 b ofthe front glass substrate 1 is flat or substantially flat. In otherwords, surface 1 b is not patterned or etched. In such embodiments, asshown in the figures, the front electrode 3 is provided on the flat orsubstantially flat surface 1 b of the glass substrate 1. Accordingly,the layers 3 a-3 f of the electrode 3 are all substantially flat orplanar in such example embodiments of this invention. Alternatively, inother example embodiments, the inner surface 1 b of the glass substrate1 may be patterned like outer surface 1 a.

In certain example embodiments of this invention, the photovoltaicdevice may be made by providing glass substrate 1, and then depositing(e.g., via sputtering or any other suitable technique) multilayerelectrode 3 on the substrate 1. Thereafter the structure includingsubstrate 1 and front electrode 3 is coupled with the rest of the devicein order to form the photovoltaic device shown in FIG. 1. For example,the semiconductor layer 5 may then be formed over the front electrode onsubstrate 1. Alternatively, the back contact 7 and semiconductor 5 maybe fabricated/formed on substrate 11 (e.g., of glass or other suitablematerial) first; then the electrode 3 and dielectric 2 may be formed onsemiconductor 5 and encapsulated by the substrate 1 via an adhesive suchas EVA.

The alternating nature of the TCO layers 3 a, 3 c and/or 3 e, and theconductive substantially metallic IR reflecting layers 3 b and/or 3 d,is also advantageous in that it also one, two, three, four or all of thefollowing advantages to be realized: (a) reduced sheet resistance(R_(s)) of the overall electrode 3 and thus increased conductivity andimproved overall photovoltaic module output power; (b) increasedreflection of infrared (IR) radiation by the electrode 3 therebyreducing the operating temperature of the semiconductor 5 portion of thephotovoltaic module so as to increase module output power; (c) reducedreflection and increased transmission of light in the visible region offrom about 450-700 nm (and/or 450-600 nm) by the front electrode 3 whichleads to increased photovoltaic module output power; (d) reduced totalthickness of the front electrode coating 3 which can reduce fabricationcosts and/or time; (e) an improved or enlarged process window in formingthe TCO layer(s) because of the reduced impact of the TCO's conductivityon the overall electric properties of the module given the presence ofthe highly conductive substantially metallic layer(s); and/or (f)reduced risk of thermal stress caused module breakage by reflectingsolar thermal energy and reducing temperature difference across themodule.

The active semiconductor region or film 5 may include one or morelayers, and may be of any suitable material. For example, the activesemiconductor film 5 of one type of single junction amorphous silicon(a-Si) photovoltaic device includes three semiconductor layers, namely ap-layer, an n-layer and an i-layer. The p-type a-Si layer of thesemiconductor film 5 may be the uppermost portion of the semiconductorfilm 5 in certain example embodiments of this invention; and the i-layeris typically located between the p and n-type layers. These amorphoussilicon based layers of film 5 may be of hydrogenated amorphous siliconin certain instances, but may also be of or include hydrogenatedamorphous silicon carbon or hydrogenated amorphous silicon germanium,hydrogenated microcrystalline silicon, or other suitable material(s) incertain example embodiments of this invention. It is possible for theactive region 5 to be of a double-junction or triple-junction type inalternative embodiments of this invention. CdTe may also be used forsemiconductor film 5 in alternative embodiments of this invention.

Back contact, reflector and/or electrode 7 may be of any suitableelectrically conductive material. For example and without limitation,the back contact or electrode 7 may be of a TCO and/or a metal incertain instances. Example TCO materials for use as back contact orelectrode 7 include indium zinc oxide, indium-tin-oxide (ITO), tinoxide, and/or zinc oxide which may be doped with aluminum (which may ormay not be doped with silver). The TCO of the back contact 7 may be ofthe single layer type or a multi-layer type in different instances.Moreover, the back contact 7 may include both a TCO portion and a metalportion in certain instances. For example, in an example multi-layerembodiment, the TCO portion of the back contact 7 may include a layer ofa material such as indium zinc oxide (which may or may not be doped withsilver), indium-tin-oxide (ITO), tin oxide, and/or zinc oxide closest tothe active region 5, and the back contact may include another conductiveand possibly reflective layer of a material such as silver, molybdenum,platinum, steel, iron, niobium, titanium, chromium, bismuth, antimony,or aluminum further from the active region 5 and closer to thesuperstrate 11. The metal portion may be closer to superstrate 11compared to the TCO portion of the back contact 7.

The photovoltaic module may be encapsulated or partially covered with anencapsulating material such as encapsulant 9 in certain exampleembodiments. An example encapsulant or adhesive for layer 9 is EVA orPVB. However, other materials such as Tedlar type plastic, Nuvasil typeplastic, Tefzel type plastic or the like may instead be used for layer 9in different instances.

Utilizing the highly conductive substantially metallic IR reflectinglayers 3 b and 3 d, and TCO layers 3 a, 3 c and 3 d, to form amultilayer front electrode 3, permits the thin film photovoltaic deviceperformance to be improved by reduced sheet resistance (increasedconductivity) and tailored reflection and transmission spectra whichbest fit photovoltaic device response. Refractive indices of glass 1,hydrogenated a-Si as an example semiconductor 5, Ag as an example forlayers 3 b and 3 d, and an example TCO are shown in FIG. 2. Based onthese refractive indices (n), predicted transmission spectra impinginginto the semiconductor 5 from the incident surface of substrate 1 areshown in FIG. 3. In particular, FIG. 3 is a percent transmission (T %)versus wavelength (nm) graph illustrating transmission spectra into ahydrogenated Si thin film 5 of a photovoltaic device comparing Examples1-3 of this invention (see Examples 1-3 in FIGS. 5-7) versus acomparative example (TCO reference). The TCO reference was made up of 3mm thick glass substrate 1 and from the glass outwardly 30 nm of tinoxide, 20 nm of silicon oxide and 350 nm of TCO. FIG. 3 thus shows thatthe examples of this invention (Examples 1-3 shown in FIGS. 5-7) hasincreased transmission in the approximately 450-600 and 450-700 nmwavelength ranges and thus increased photovoltaic module output power,compared to the comparative example (TCO reference).

Example 1 shown in FIG. 5 and charted in FIGS. 3-4 was made up of 3 mmthick glass substrate 1, 16 nm thick TiO₂ dielectric layer 2, 10 nmthick zinc oxide TCO doped with Al 3 a, 8 nm thick Ag IR reflectinglayer 3 b, and 115 nm thick zinc oxide TCO doped with Al 3 e. Surface 1a was flat in this example. Layers 3 c, 3 d and 3 f were not present inExample 1. Example 2 shown in FIG. 6 and charted in FIGS. 3-4 was madeup of 3 mm thick glass substrate 1 with a flat surface 1 a, 16 nm thickTiO₂ dielectric layer 2, 10 nm thick zinc oxide TCO doped with Al 3 a, 8nm thick Ag IR reflecting layer 3 b, 100 nm thick zinc oxide TCO dopedwith Al 3 e, and 20 nm thick titanium suboxide layer 3 f. Example 3shown in FIG. 7 and charted in FIGS. 3-4 was made up of 3 mm thick glasssubstrate 1 with a flat surface 1 a, 45 nm thick dielectric layer 2, 10nm thick zinc oxide TCO doped with Al 3 a, 5 nm thick Ag IR reflectinglayer 3 b, 75 nm thick zinc oxide TCO doped with Al 3 c, 7 nm thick AgIR reflecting layer 3 d, 95 nm thick zinc oxide TCO doped with Al 3 e,and 20 nm thick titanium suboxide layer 3 f. These single anddouble-silver layered coatings of Examples 1-3 had a sheet resistanceless than 10 ohms/square and 6 ohms/square, respectively, and totalthicknesses much less than the 400 nm thickness of the prior art.Examples 1-3 had tailored transmission spectra, as shown in FIG. 3,having more than 80% transmission into the semiconductor 5 in part orall of the wavelength range of from about 450-600 nm and/or 450-700 nm,where AM1.5 has the strongest intensity and photovoltaic devices maypossibly have the highest or substantially the highest quantumefficiency.

Meanwhile, FIG. 4 is a percent reflection (R %) versus wavelength (nm)graph illustrating reflection spectra from a hydrogenated Si thin filmof a photovoltaic device comparing Examples 1-3 versus the abovementioned comparative example; this shows that Examples 1-3 hadincreased reflection in the IR range thereby reducing the operatingtemperature of the photovoltaic modules so as to increase module outputpower, compared to the comparative example. In FIG. 4, the lowreflection in the visible range of from about 450-600 nm and/or 450-700nm (the cell's high efficiency range) is advantageously coupled withhigh reflection in the near and short IR range beyond about 1000 nm; thehigh reflection in the near and short IR range reduces the absorption ofsolar thermal energy that will result in a better cell output due to thereduced cell temperature and series resistance in the module. As shownin FIG. 4, the front glass substrate 1 and front electrode 3 takentogether have a reflectance of at least about 45% (more preferably atleast about 55%) in a substantial part or majority of a near to short IRwavelength range of from about 1000-2500 nm and/or 1000 to 2300 nm. Incertain example embodiments, it refelects at least 50% of solar energyin the range of from 1000-2500 nm and/or 1200-2300 nm. In certainexample embodiments, the front glass substrate and front electrode 3taken together have an IR reflectance of at least about 45% and/or 55%in a substantial part or a majority of a near IR wavelength range offrom about 1000-2500 nm, possibly from 1200-2300 nm. In certain exampleembodiments, it may block at least 50% of solar energy in the range of1000-2500 nm.

While the electrode 3 is used as a front electrode in a photovoltaicdevice in certain embodiments of this invention described andillustrated herein, it is also possible to use the electrode 3 asanother electrode in the context of a photovoltaic device or otherwise.

FIG. 8 is a cross sectional view of a photovoltaic device according toanother example embodiment of this invention. An optional antireflective(AR) layer (not shown) may be provided on the light incident side of thefront glass substrate 1 in any embodiment of this invention. Thephotovoltaic device in FIG. 8 includes glass substrate 1, dielectriclayer(s) 2 (e.g., of or including one or more of silicon oxide, siliconoxynitride, silicon nitride, titanium oxide, niobium oxide, and/or thelike) which may function as a sodium barrier for blocking sodium frommigrating out of the front glass substrate 1, seed layer 4 b (e.g., ofor including zinc oxide, zinc aluminum oxide, tin oxide, tin antimonyoxide, indium zinc oxide, or the like) which may be a TCO or dielectricin different example embodiments, silver based IR reflecting layer 4 c,optional overcoat or contact layer 4 d (e.g., of or including an oxideof Ni and/or Cr, zinc oxide, zinc aluminum oxide, or the like) which maybe a TCO, TCO 4 e (e.g., of or including zinc oxide, zinc aluminumoxide, tin oxide, tin antimony oxide, zinc tin oxide, indium tin oxide,indium zinc oxide, or the like), optional buffer layer 4 f (e.g., of orincluding zinc oxide, zinc aluminum oxide, tin oxide, tin antimonyoxide, zinc tin oxide, indium tin oxide, indium zinc oxide, or the like)which may be conductive to some extent, semiconductor 5 (e.g., CdS/CdTe,a-Si, or the like), optional back contact, reflector and/or electrode 7,optional adhesive 9, and optional back glass substrate 11. It is notedthat in certain example embodiments, layer 4 b may be the same as layer3 a described above, layer 4 c may be the same as layer 3 b or 3 ddescribed above this applies to FIGS. 8-10), layer 4 e may be the sameas layer 3 e described above (this also applies to FIGS. 8-10), andlayer 4 f may be the same as layer 3 f described above (this alsoapplies to FIGS. 8-10) (see descriptions above as to other embodimentsin this respect). Likewise, layers 1, 5, 7, 9 and 11 are also discussedabove in connection with other embodiments, as are surfaces 1 a and 1 bof the front glass substrate 1.

For purposes of example only, an example of the FIG. 8 embodiment is asfollows (note that certain optional layers shown in FIG. 8 are not usedin this example). For example, referring to FIG. 8, glass substrate 1(e.g., about 3.2 mm thick), dielectric layer 2 (e.g., silicon oxynitrideabout 20 nm thick possibly followed by dielectric TiOx about 20 nmthick), Ag seed layer 4 b (e.g., dielectric or TCO zinc oxide or zincaluminum oxide about 10 nm thick), IR reflecting layer 4 c (silver about5-8 nm thick), TCO 4 e (e.g., conductive zinc oxide, tin oxide, zincaluminum oxide, ITO from about 50-250 nm thick, more preferably fromabout 100-150 nm thick), and possibly conductive buffer layer 4 f (TCOzinc oxide, tin oxide, zinc aluminum oxide, ITO, or the like, from about10-50 nm thick). In certain example embodiments, the buffer layer 4 f(or 3 f) is designed to have a refractive index (n) of from about 2.1 to2.4, more preferably from about 2.15 to 2.35, for substantial indexmatching to the semiconductor 5 (e.g., CdS or the like) in order toimprove efficiency of the device.

The photovoltaic device of FIG. 8 may have a sheet resistance of nogreater than about 18 ohms/square, more preferably no grater than about15 ohms/square, even more preferably no greater than about 13ohms/square in certain example embodiments of this invention. Moreover,the FIG. 8 embodiment may have tailored transmission spectra having morethan 80% transmission into the semiconductor 5 in part or all of thewavelength range of from about 450-600 nm and/or 450-700 nm, where AM1.5may have the strongest intensity and in certain example instances thecell may have the highest or substantially the highest quantumefficiency.

FIG. 9 is a cross sectional view of a photovoltaic device according toyet another example embodiment of this invention. The photovoltaicdevice of the FIG. 9 embodiment includes optional antireflective (AR)layer (not shown) on the light incident side of the front glasssubstrate 1, first dielectric layer 2 a, second dielectric layer 2 b,third dielectric layer 2 c which may optionally function as a seed layer(e.g., of or including zinc oxide, zinc aluminum oxide, tin oxide, tinantimony oxide, indium zinc oxide, or the like) for the silver basedlayer 4 c, conductive silver based IR reflecting layer 4 c, optionalovercoat or contact layer 4 d (e.g., of or including an oxide of Niand/or Cr, zinc oxide, zinc aluminum oxide, or the like) which may be aTCO or dielectric, TCO 4 e (e.g., including one or more layers, such asof or including zinc oxide, zinc aluminum oxide, tin oxide, tin antimonyoxide, zinc tin oxide, indium tin oxide, indium zinc oxide, or thelike), optional conductive buffer layer 4 f (e.g., of or including zincoxide, zinc aluminum oxide, tin oxide, tin antimony oxide, zinc tinoxide, indium tin oxide, indium zinc oxide, or the like), semiconductor5 (e.g., one or more layers such as CdS/CdTe, a-Si, or the like),optional back contact, reflector and/or electrode 7, optional adhesive9, and optional back/rear glass substrate 11. Layers 4 e and 4 f arepreferably conductive in order to ensure that the metal layer 4 c can beused in connection with the absorber film 5 to generate charge.Semiconductor film 5 may include a single pin or pn semiconductorstructure, or a tandem semiconductor structure in different embodimentsof this invention. Semiconductor 5 may be of or include silicon incertain example instances. In other example embodiments, semiconductorfilm 5 may include a first layer of or including CdS (e.g., windowlayer) adjacent or closest to layer(s) 4 e and/or 4 f and a secondsemiconductor layer of or including CdTe (e.g., main absorber) adjacentor closest to the back electrode or contact 7.

Referring to the FIG. 9 embodiment (and the FIG. 10 embodiment), incertain example embodiments, first dielectric layer 2 a has a relativelylow refractive index (n) (e.g., n of from about 1.7 to 2.2, morepreferably from about 1.8 to 2.2, still more preferably from about 1.95to 2.1, and most preferably from about 2.0 to 2.08), second dielectriclayer 2 b has a relatively high (compared to layer 2 a) refractive index(n) (e.g., n of from about 2.2 to 2.6, more preferably from about 2.3 to2.5, and most preferably from about 2.35 to 2.45), and third dielectriclayer 2 c has a relatively low (compared to layer 2 b) refractive index(n) (e.g., n of from about 1.8 to 2.2, more preferably from about 1.95to 2.1, and most preferably from about 2.0 to 2.05). In certain exampleembodiments, the first low index dielectric layer 2 a may be of orinclude silicon nitride, silicon oxynitride, or any other suitablematerial, the second high index dielectric layer 2 b may be of orinclude an oxide of titanium (e.g., TiO₂ or the like), and the thirddielectric layer 2 c may be of or include zinc oxide or any othersuitable material. In certain example embodiments, layers 2 a-2 ccombine to form a good index matching stack which also functions as abuffer against sodium migration from the glass 1. In certain exampleembodiments, the first dielectric layer 2 a is from about 5-30 nm thick,more preferably from about 10-20 nm thick, the second dielectric layer 2b is from about 5-30 nm thick, more preferably from about 10-20 nmthick, and the third layer 2 c is of a lesser thickness and is fromabout 3-20 nm thick, more preferably from about 5-15 nm thick, and mostpreferably from about 6-14 nm thick. While layers 2 a, 2 b and 2 c aredielectrics in certain embodiments of this invention, one, two or allthree of these layers may be dielectric or TCO in certain other exampleembodiments of this invention. Layers 2 b and 2 c are metal oxides incertain example embodiments of this invention, whereas layer 2 a is ametal oxide and/or nitride, or silicon nitride in certain exampleinstances. Layers 2 a-2 c may be deposited by sputtering or any othersuitable technique.

Still referring to the FIG. 9 embodiment (and the FIG. 10-11embodiments), the TCO layer(s) 4 e may be of or include any suitable TCOincluding but not limited to zinc oxide, zinc aluminum oxide, tin oxideand/or the like. TCO layer or file 4 e may include multiple layers incertain example instances. For example, certain instances, the TCO 4includes a first layer of a first TCO metal oxide (e.g., zinc oxide)adjacent Ag 4 c, Ag overcoat 4 d and a second layer of a second TCOmetal oxide (e.g., tin oxide) adjacent and contacting layer 4 f and/or5.

For purposes of example only, an example of the FIG. 9 embodiment is asfollows. For example, referring to FIG. 9, glass substrate 1 (e.g.,float glass about 3.2 mm thick, and a refractive index n of about 1.52),first dielectric layer 2 a (e.g., silicon nitride about 15 nm thick,having a refractive index n of about 2.07), second dielectric layer 2 b(e.g., oxide of Ti, such as TiO₂ or other suitable stoichiometry, about16 nm thick, having a refractive index n of about 2.45), thirddielectric layer 2 c (e.g., zinc oxide, possibly doped with Al, about 9nm thick, having a refractive index n of about 2.03), IR reflectinglayer 4 c (silver about 3-9 nm thick, e.g., 6 nm), silver overcoat 4 dof NiCrO_(x) about 1-3 nm thick which may or may not be oxidationgraded, TCO film 4 e (e.g., conductive zinc oxide, zinc aluminum oxideand/or tin oxide about 10-150 nm thick), a semiconductor film 5including a first layer of CdS (e.g., about 70 nm) closest to substrate1 and a second layer of CdTe further from substrate 1, back contact orelectrode 7, optional adhesive 9, and optionally substrate 11.

The photovoltaic device of FIG. 9 (and/or FIGS. 10-11) may have a sheetresistance of no greater than about 18 ohms/square, more preferably nograter than about 15 ohms/square, even more preferably no greater thanabout 13 ohms/square in certain example embodiments of this invention.Moreover, the FIG. 9 (and/or FIGS. 10-11) embodiment may have tailoredtransmission spectra having more than 80% transmission into thesemiconductor 5 in part or all of the wavelength range of from about450-600 nm and/or 450-700 nm, where AM1.5 may have the strongestintensity.

FIG. 10 is a cross sectional view of a photovoltaic device according tostill another example embodiment of this invention. The FIG. 10embodiment is the same as the FIG. 9 embodiment discussed above, exceptfor the TCO film 4 e. In the FIG. 10 embodiment, the TCO film 4 eincludes a first layer 4 e′ of or including a first TCO metal oxide(e.g., zinc oxide, which may or may not be doped with Al or the like)adjacent and contacting layer 4 d and a second layer 4 e″ of a secondTCO metal oxide (e.g., tin oxide) adjacent and contacting layer 4 fand/or 5 (e.g., layer 4 f may be omitted, as in previous embodiments).Layer 4 e′ is also substantially thicker than layer 4 e″ in certainexample embodiments. In certain example embodiments, the first TCO layer4 e′ has a resistivity which is less than that of the second TCO layer 4e″. In certain example embodiments, the first TCO layer 4 e′ may be ofzinc oxide, Al-doped zinc oxide, or ITO about 70-150 nm thick (e.g.,about 110 nm) having a resistivity of no greater than about 1 ohm·cm,and the second TCO layer 4 e″ may be of tin oxide about 10-50 nm thick(e.g., about 30 nm) having a resistivity of from about 10-100 ohm·cm,possibly from about 2-100 ohm·cm. The first TCO layer 4 e′ is thickerand more conductive than the second TCO layer 4 e″ in certain exampleembodiments, which is advantageous as layer 4 e′ is closer to theconductive Ag based layer 4 c thereby leading to improved efficiency ofthe photovoltaic device. Moreover, this design is advantageous in thatCdS of the film 5 adheres or sticks well to tin oxide which may be usedin or for layer 4 e″. TCO layers 4 e′ and/or 4 e″ may be deposited bysputtering or any other suitable technique.

In certain example instances, the first TCO layer 4 e′ may be of orinclude ITO (indium tin oxide) instead of zinc oxide. In certain exampleinstances, the ITO of layer 4 e′ may be about 90% In, 10% Sn, oralternatively about 50% In, 50% Sn.

The use of at least these three dielectrics 2 a-2 c is advantageous inthat it permits reflections to be reduced thereby resulting in a moreefficient photovoltaic device. Moreover, it is possible for the overcoatlayer 4 d (e.g., of or including an oxide of Ni and/or Cr) to beoxidation graded, continuously or discontinuously, in certain exampleembodiments of this invention. In particular, layer 4 d may be designedso as to be more metallic (less oxided) at a location therein closer toAg based layer 4 d than at a location therein further from the Ag basedlayer 4 d; this has been found to be advantageous for thermal stabilityreasons in that the coating does not degrade as much during subsequentlyhigh temperature processing which may be associated with thephotovoltaic device manufacturing process or otherwise.

In certain example embodiments of this invention, it has beensurprisingly found that a thickness of from about 120-160 nm, morepreferably from about 130-150 nm (e.g., 140 nm), for the TCO film 4 e isadvantageous in that the Jsc peaks in this range. For thinner TCOthicknesses, the Jsc decreases by as much as about 6.5% until it bottomsout at about a TCO thickness of about 60 nm. Below 60 nm, it increasesagain until at a TCO film 4 e thickness of about 15-35 nm (morepreferably 20-30 nm) it is attractive, but such thin coatings may not bedesirable in certain example non-limiting situations. Thus, in order toachieve a reduction in short circuit current density of CdS/CdTephotovoltaic devices in certain example instances, the thickness of TCOfilm 4 e may be provided in the range of from about 15-35 nm, or in therange of from about 120-160 nm or 130-150 nm.

FIG. 11 is a cross sectional view of a photovoltaic device according tostill another example embodiment of this invention. The FIG. 11embodiment is similar to the FIG. 9-10 embodiments discussed above,except for the differences shown in the figure. FIG. 11 is a crosssectional view of a photovoltaic device according to an exampleembodiment of this invention. The photovoltaic device of the FIG. 11 mayinclude: optional antireflective (AR) layer (not shown) on the lightincident side of the front glass substrate 1; first dielectric layer 2 aof or including one or more of silicon nitride (e.g., Si₃N₄ or othersuitable stoichiometry), silicon oxynitride, silicon oxide (e.g., SiO₂or other suitable stoichiometry), and/or tin oxide (e.g., SnO₂ or othersuitable stoichiometry); second dielectric layer 2 b of or includingtitanium oxide (e.g., TiO₂ or other suitable stoichiometry) and/orniobium oxide; third layer 2 c (which may be a dielectric or a TCO)which may optionally function as a seed layer (e.g., of or includingzinc oxide, zinc aluminum oxide, tin oxide, tin antimony oxide, indiumzinc oxide, or the like) for the silver based layer 4 c; conductivesilver based IR reflecting layer 4 c; overcoat or contact layer 4 d(which may be a dielectric or conductive) of or including an oxide of Niand/or Cr, NiCr, Ti, an oxide of Ti, zinc aluminum oxide, or the like;TCO 4 e (e.g., including one or more layers) of or including zinc oxide,zinc aluminum oxide, tin oxide, tin antimony oxide, zinc tin oxide,indium tin oxide (ITO), indium zinc oxide, and/or zinc gallium aluminumoxide; optional buffer layer 4 f which may be a TCO in certain instances(e.g., of or including zinc oxide, zinc aluminum oxide, tin oxide, tinantimony oxide, zinc tin oxide, indium tin oxide, indium zinc oxide,titanium oxide, or the like) and which may be conductive to some extent;semiconductor film 5 of or including one or more layers such asCdS/CdTe, a-Si, or the like (e.g., film 5 may be made up of a layer ofor including CdS adjacent layer 4 f, and a layer of or including CdTeadjacent layer 7); optional back contact/electrode/reflector 7 ofaluminum or the like; optional adhesive 9 of or including a polymer suchas PVB; and optional back/rear glass substrate 11. In certain exampleembodiments of this invention, dielectric layer 2 a may be from about10-20 nm thick, more preferably from about 12-18 nm thick; layer 2 b maybe from about 10-20 nm thick, more preferably from about 12-18 nm thick;layer 2 c may be from about 5-20 nm thick, more preferably from about5-15 nm thick (layer 2 c is thinner than one or both of layers 2 a and 2b in certain example embodiments); layer 4 c may be from about 5-20 nmthick, more preferably from about 6-10 nm thick; layer 4 d may be fromabout 0.2 to 5 nm thick, more preferably from about 0.5 to 2 nm thick;TCO film 4 e may be from about 50-200 nm thick, more preferably fromabout 75-150 nm thick, and may have a resistivity of no more than about100 mΩ in certain example instances; and buffer layer 4 f may be fromabout 10-50 nm thick, more preferably from about 20-40 nm thick and mayhave a resistivity of no more than about 1 MΩ-cm in certain exampleinstances. Moreover, the surface of glass 1 closest to the sun may bepatterned via etching or the like in certain example embodiments of thisinvention.

Optional buffer layer 4 f may provide substantial index matching betweenthe semiconductor film 5 (e.g., CdS portion) to the TCO 4 e in certainexample embodiments, in order to optimize total solar transmissionreaching the semiconductor.

Still referring to the FIG. 11 embodiments, semiconductor film 5 mayinclude a single pin or pn semiconductor structure, or a tandemsemiconductor structure in different embodiments of this invention.Semiconductor 5 may be of or include silicon in certain exampleinstances. In other example embodiments, semiconductor film 5 mayinclude a first layer of or including CdS (e.g., window layer) adjacentor closest to layer(s) 4 e and/or 4 f and a second semiconductor layerof or including CdTe (e.g., main absorber) adjacent or closest to theback electrode or contact 7.

Also referring to FIG. 11, in certain example embodiments, firstdielectric layer 2 a has a relatively low refractive index (n) (e.g., nof from about 1.7 to 2.2, more preferably from about 1.8 to 2.2, stillmore preferably from about 1.95 to 2.1, and most preferably from about2.0 to 2.08), second dielectric layer 2 b has a relatively high(compared to layer 2 a) refractive index (n) (e.g., n of from about 2.2to 2.6, more preferably from about 2.3 to 2.5, and most preferably fromabout 2.35 to 2.45), and third dielectric layer 2 c may optionally havea relatively low (compared to layer 2 b) refractive index (n) (e.g., nof from about 1.8 to 2.2, more preferably from about 1.95 to 2.1, andmost preferably from about 2.0 to 2.05). In certain example embodiments,layers 2 a-2 c combine to form a good index matching stack forantireflection purposes and which also functions as a buffer againstsodium migration from the glass 1. In certain example embodiments, thefirst dielectric layer 2 a is from about 5-30 nm thick, more preferablyfrom about 10-20 nm thick, the second dielectric layer 2 b is from about5-30 nm thick, more preferably from about 10-20 nm thick, and the thirdlayer 2 c is of a lesser thickness and is from about 3-20 nm thick, morepreferably from about 5-15 nm thick, and most preferably from about 6-14nm thick. While layers 2 a, 2 b and 2 c are dielectrics in certainembodiments of this invention, one, two or all three of these layers maybe dielectric or TCO in certain other example embodiments of thisinvention. Layers 2 b and 2 c are metal oxides in certain exampleembodiments of this invention, whereas layer 2 a is a metal oxide and/ornitride, or silicon nitride in certain example instances. Layers 2 a-2 cmay be deposited by sputtering or any other suitable technique.

Still referring to the FIG. 11 embodiment, the TCO layer(s) 4 e may beof or include any suitable TCO including but not limited to zinc oxide,zinc aluminum oxide, tin oxide and/or the like. TCO layer or file 4 emay include multiple layers in certain example instances. For example,certain instances, the TCO 4 includes a first layer of a first TCO metaloxide (e.g., zinc oxide) adjacent Ag 4 c, Ag overcoat 4 d and a secondlayer of a second TCO metal oxide (e.g., tin oxide) adjacent andcontacting layer 4 f and/or 5. The photovoltaic device of FIG. 11 mayhave a sheet resistance of no greater than about 18 ohms/square, morepreferably no grater than about 15 ohms/square, even more preferably nogreater than about 13 ohms/square in certain example embodiments of thisinvention. Moreover, the FIG. 11 embodiment may have tailoredtransmission spectra having more than 80% transmission into thesemiconductor 5 in part or all of the wavelength range of from about450-600 nm and/or 450-700 nm, where AM1.5 may have the strongestintensity, in certain example embodiments of this invention.

Examples 4-5 are discussed below, and each have a textured surface 1 aof the front glass substrate 1 as shown in the figures herein. InExample 4, outer surface 1 a of the front transparent glass substrate 1was lightly etched having fine features that in effect function as asingle layered low index antireflection coating suitable for, e.g., CdTesolar cell applications. Example 5 had larger features on the texturedsurface 1 a of the front glass substrate, again formed by etching, thattrap incoming light and refracts light into the semiconductor at obliqueangles, suitable for, e.g., a-Si single and/or tandem solar cellapplications. The interior surface 1 b of the glass substrate 1 was flatin each of Examples 4 and 5, as was the front electrode 3.

In Example 4, referring to FIG. 11, the layer stack moving from theglass 1 inwardly toward the semiconductor 5 was glass 1, silicon nitride(15 nm thick) layer 2 a, TiO_(x) (16 nm thick) layer 2 b, ZnAlO_(x) (10nm thick) layer 2 c, Ag (7 nm thick) layer 4 c, NiCrO_(x) (1 nm thick)layer 4 d, ITO (110 nm thick) layer 4 e, SnO_(x) (30 nm thick) layer 4f, and then the CdS/CdTe semiconductor. The etched surface 1 a of thefront glass substrate 1 had an effective index and thickness around1.35-1.42 and 110 nm, respectively. The etched surface 1 a acted as anAR coating (although no such coating was physically present) andincreased the transmission 2-3% which will be appreciated as beinghighly advantageous, around the wavelength region from 400-1,000 nm asshown in FIGS. 12-13. As shown in FIG. 15, the combination of theAg-based TCC 3 and the textured front surface 1 a resulted in enhancedtransmission into the CdTe/CdS semiconductor film 5, especially in theregion from 500-700 nm where CdTe PV device QE and solar flux aresignificant.

FIG. 12 is a measured transmission (T) and reflection (R) (% from firstsurface 1 a) spectra, versus wavelength (nm), showing results fromExample 4, where the example used a 10 ohms/sq. Ag-based TCC coating 3 afront glass substrate 1 with a textured surface 1. As explained above,the Example 4 with the textured surface 1 a had slightly increasedtransmission (T) and slightly reduced reflection (R) in the 500-700 nmregion compared to a comparative example shown in FIG. 12 where surface1 a (first surface) was not etched. This is advantage in that morecurrent is generated in the semiconductor film 5 of the PV device. FIG.15 is a percent transmission (T %) versus wavelength (nm) graphillustrating transmission spectra into a CdS/CdTe cell of a photovoltaicdevice comparing Example 4 versus comparative examples. FIG. 15(predicted transmission into CdTe/CdS in a CdTe solar cell module ofExample 4, having different front substrates) shows that the Example 4realized increased transmission in the approximately 500-700 nmwavelength range and thus increased photovoltaic module output power,compared to the comparative example without the etched front surface (xdotted line) and the comparative example of the conventional TCOsuperstrate (o solid line).

In Example 5, referring to FIG. 11, the layer stack moving from theglass 1 inwardly toward the semiconductor 5 was glass 1, silicon nitride(15 nm thick) layer 2 a, TiO_(x) (10 nm thick) layer 2 b, ZnAlO_(x) (10nm thick) layer 2 c, Ag (8 nm thick) layer 4 c, NiCrO_(x) (1 nm thick)layer 4 d, ITO (70 nm thick) layer 4 e, SnO_(x) (20 nm thick) layer 4 f,and then the a-Si semiconductor 5. FIG. 14 shows measured and predictedresults; measured integrated and specular transmission spectra andpredicted light scattering/diffusion, according to Example 5. FIG. 14shows that the integrated transmission that includes both specular anddiffused transmission lights is around 17% higher than the specular onlytransmission light. This implies that more than 17% of light in thevisible and near-IR regions are either diffused or scattered. A diffusedand/or scattered light has increased optical path in photovoltaicmaterials 5, and is especially desired in a-Si type solar cells.

FIG. 16 is a cross sectional view of an example of the FIG. 11embodiment of this invention. Solar radiation incident on photovoltaicsolar cells includes two kinds of photons, short wavelength photonshaving energies high enough to create electron-hole pairs inphotovoltaic materials and long wavelength photons having nocontribution to electron-hole pair creation but generating thermal heatthat degrades solar cell output power. Therefore, it is desired to havea transparent conductive superstrate (or front electrode/contact) thatacts as not only a transparent conductive front contact but also a shortpass filter that allows an increased amount of photons having highenough energy (such as in visible and near infra-red regions of thespectrum) into the active region or absorber of solar cells, and whichalso blocks the rest or a significant part of the rest of the incidentsolar radiation which may be harmful or undesirable. In this way, thesolar cell output power can be improved due to reduced moduletemperature by increased IR reflection, and increased transmission invisible to near IR.

FIG. 16 is a cross sectional view illustrating an example design of sucha conductive short pass filter as a front electrode, which achieves hightransmission in visible and near IR but reflects long wavelength IRlight. The thin Ag-based substantially metal layer 4 c can be single Agor Ag alloy, or sandwiched between other metallic layer such as TiNx, Tior NiCr (not shown). The overall physical thickness of the frontelectrode in the FIG. 16 embodiment may be no more than 15 nm, andresistivity of this front electrode may be no more than 20 uohm-cm. Theconductive oxide(s) can be single or multiple layered oxides of In, Sn,Zn, and their alloys with no more than 10% dopants such as Al, Ga, Sband others in certain example embodiments. The overall conductive frontelectrode has an effective refractive index of no less than 1.85, and anoptical thickness of from about 150-200, more preferably 160-190 nm, incertain example embodiments applicable to a-Si photovoltaic devices. Theoverall conductive front electrode has an effective refractive index ofno less than 1.85, and an optical thickness of from about 240-300, morepreferably 250-290 nm, in certain example embodiments applicable toCdS/CdTe photovoltaic devices. The overall sheet resistance of theconductive oxide layer is no more than 2000 ohm/sq. in certain exampleembodiments. The conductive oxide and Ag provide the required conductivepath for electron/hole pairs generated from photovoltaic materials(e.g., semiconductor 5). The transparent base layer (2 a, 2 b) can besingle or multiple layered oxide, oxynitride, or nitride, and it may beconductive or non-conductive. The transparent base layer (2 a, 2 b) hasan overall effective index no less than 2.0 and optical thickness around50-90 nm in certain example embodiments applicable to a-Si photovoltaicdevices; and may have an overall effective index no less than 2.0 andoptical thickness around 60-100 nm in certain example embodimentsapplicable to CdS/CdTe photovoltaic devices.

In certain example embodiments, the physical and/or optical thickness oflayer 4 e is at least two times thicker than that of layer 4 f, morepreferably at least 3 times thicker. Moreover, in certain exampleembodiments in connection with the FIG. 16 or other embodiments therein,layer 4 f may have a refractive index (n) of from about 1.9 to 2.1,layer 4 e may have a refractive index less than that of layer 4 f, layer4 e may have a refractive index (n) of from about 1.8 to 2.0, layer 2 cmay have a refractive index of from about 1.8 to 2.05, more preferablyfrom about 1.85 to 1.95, layer 2 b may have a refractive index of fromabout 2.1 to 2.5, more preferably from about 2.25 to 2.45, and layer 2 amay have a refractive index (n) of from about 1.9 to 2.1.

Examples 6-7 are set forth below, with reference to the FIG. 16embodiment of this invention.

Example 6 relates to an a-Si based photovoltaic device. In Example 6,referring to FIG. 16 and using physical thickness values and refractiveindex values at about 550 nm, the layer stack moving from the glass 1inwardly toward the semiconductor absorber film 5 was glass 1, siliconnitride (about 15 nm thick, refractive index “n” of about 2.0) layer 2a, TiO_(x) (about 10 nm thick, refractive index “n” of about 2.4) layer2 b, ZnAlO_(x) (about 10 nm thick, refractive index “n” of about 1.9)layer 2 c, Ag (about 8 nm thick) layer 4 c, NiCrO_(x) (about 1 nm thick)layer 4 d, TCO ITO (indium tin oxide) (about 70 nm thick, refractiveindex “n” of about 1.9) layer 4 e, TCO SnO_(x) (about 20 nm thick,refractive index “n” of about 2.0) layer 4 f, and then the a-Si basedsemiconductor 5. FIG. 17 shows the predicted results of transmissioninto the a-Si cell of this Example 6 device compared to a conventionalfront electrode consisting only of a tin oxide TCO on the glasssubstrate. In particular, FIG. 17 is a percent transmission (T %) versuswavelength (nm) graph illustrating transmission spectra for Example 6into an a-Si cell, illustrating that Example 6 (e.g., T-9 curve in FIG.17) realized increased transmission in the approximately 500-700 nmwavelength range and thus increased photovoltaic module output power,compared to the comparative example (X-marked curve in FIG. 17).Moreover, the FIG. 16-17 embodiment may have tailored transmissionspectra having more than 80%, 85% or even 87% transmission into thesemiconductor 5 in part of, the majority of, or all of the wavelengthrange of from about 450-600 nm and/or 450-700 nm, as shown in FIG. 17,in certain example embodiments of this invention.

Example 7 relates to a CdS/CdTe based photovoltaic device. In Example 7,referring to FIG. 16 and using physical thickness values and refractiveindex values at about 550 nm, the layer stack moving from the glass 1inwardly toward the semiconductor absorber film 5 was glass 1, siliconnitride (about 15 nm thick, refractive index “n” of about 2.0) layer 2a, TiO_(x) (about 16 nm thick, refractive index “n” of about 2.4) layer2 b, ZnAlO_(x) (about 10 nm thick, refractive index “n” of about 1.9)layer 2 c, Ag (about 7 nm thick) layer 4 c, NiCrO_(x) (about 1 nm thick)layer 4 d, TCO ITO (indium tin oxide) (about 110 nm thick, refractiveindex “n” of about 1.9) layer 4 e, TCO SnO_(x) (about 30 nm thick,refractive index “n” of about 2.0) layer 4 f, and then the a-Si basedsemiconductor 5. FIG. 18 shows the predicted results of incidenttransmission into the CdS/CdTe cell of this Example 7 device compared toa similar device including a conventional front electrode consistingonly of a tin oxide TCO on the glass substrate. In particular, FIG. 18is a percent transmission (T %) versus wavelength (nm) graphillustrating transmission spectra for Example 7 into a CdS/CdTe cell,illustrating that Example 7 (e.g., T-Ag curve in FIG. 18) realizedincreased transmission in the approximately 450-600 and 500-700 nmwavelength ranges and thus increased photovoltaic module output power,compared to the comparative example (X-marked curve in FIG. 18).Moreover, the FIG. 16, 18 embodiment may have tailored transmissionspectra having more than 85%, 90% or even 91 or 92% transmission intothe semiconductor 5 in part of, the majority of, or all of thewavelength range(s) of from about 500-600 nm, 450-600 nm and/or 450-700nm, as shown in FIG. 18, in certain example embodiments of thisinvention.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1-13. (canceled)
 14. A front electrode structure for a photovoltaic device, the front electrode structure comprising: a front substantially transparent glass substrate; a first layer comprising one or more of silicon nitride, silicon oxide, silicon oxynitride, and/or tin oxide; a second layer comprising metal oxide and having a higher refractive index (n) than the first layer, wherein at least the first layer is located between the front substrate and the second layer; a third layer comprising metal oxide; a fourth layer that is conductive and comprises silver, wherein at least the third layer is provided between the conductive layer comprising silver and the second layer; a fifth layer that is a transparent conductive oxide (TCO) layer comprising metal oxide between the conductive layer comprising silver and a sixth layer that is a transparent conductive oxide (TCO) layer comprising metal oxide; wherein the fifth layer has a thickness substantially greater than that of the sixth layer, and wherein a layer stack comprising said first, second, third, fourth, fifth and sixth layers is provided on an interior surface of the front glass substrate facing the semiconductor film of the photovoltaic device.
 15. The front electrode structure for a photovoltaic device of claim 14, wherein an exterior surface of the glass substrate is etched, but the interior surface of the glass substrate on which the layer stack is provided is not etched and is substantially flat.
 16. The front electrode structure for a photovoltaic device of claim 14, wherein the front electrode structure acts not only as a front electrode for the photovoltaic device, but also as a short pass filter that (i) allows a large amount of photons having energy in the visible and near-IR regions into the semiconductor film, and (ii) blocks significant amounts of other IR radiation from reaching the semiconductor film.
 17. The front electrode structure for a photovoltaic device of claim 14, wherein the semiconductor film comprises a-Si, and the front electrode structure has a transmission of at least 85% in at least a majority of a range of from 450-700 nm.
 18. The front electrode structure for a photovoltaic device of claim 14, wherein the semiconductor film comprises a-Si, and the front electrode structure has a transmission of at least 87% in at least part of a range of from 450-700 nm.
 19. The front electrode structure for a photovoltaic device of claim 14, wherein the semiconductor film comprises CdS and/or CdTe, and the front electrode structure has a transmission of at least 90% in at least a majority of a range of from 500-600 nm
 20. The front electrode structure for a photovoltaic device of claim 14, wherein the semiconductor film comprises CdS and/or CdTe, and the front electrode structure has a transmission of at least 92% in at least part of a range of from 500-600 nm.
 21. The front electrode structure for a photovoltaic device of claim 14, wherein the front substrate and all layers of the photovoltaic device on a front side of the semiconductor film taken together have an IR reflectance of at least about 45% in at least a substantial part of an IR wavelength range of from about 1400-2300 nm.
 22. A photovoltaic device including the front electrode structure of claim
 14. 